Recently, wired serial data transmission technologies have been developed and a transmission speed of several tens of gigabits per second (Gb/s) is to be put into practical use in electric transmission lines using metal materials. In the data transmission system, an influence of inter-symbol interference (ISI) should be considered.
The ISI is a phenomenon which results from a delay of a signal wave proceeding in a transmission line and a reflection on a transmission line end. In the ISI, a traveling wave and a reflective wave overlap and a transmission waveform collapses. If the ISI occurs, a probability of erroneously determining a signal level (0, 1) at a receiving end is increased. A bit error rate which is an index of communication conditions becomes thereby worse.
To cancel the ISI, a decision feedback equalizer (DFE) is effective. Specifications of the DFE are designed into PCI Express (registered trademark) (hereinafter abbreviated as PCIe), too, in and after the third generation.
The above-explained DFE operates effectively by adaptively updating a tap coefficient for input data. For this reason, the DFE may make an operation error and give an influence to data communications, during a no-signal period in which no input data is present.